Autofocus can be realized by using the principle of light reflection on an object, and accepting reflected light from the object by a sensor CCD (charge-coupled device) of a camera, and driving an electric focusing device to automatically focus under the control of a computer. A focusing method by using contrast ratio can realize autofocus by detecting the contour edge of an image. The clearer the contour edge of the image is, the greater the brightness gradient is, or the greater the contrast ratio between the object of edge and the background of the image is. On the contrary, the contour edge of an out-of-focus image blurs, and the brightness gradient or the contrast ratio descends. The farther the out-of-focus is, the lower the contrast ratio is.
An existing single-camera focusing system can realize autofocus on the basis of a VCM (voice coil motor) gradually moving from an initial focus to a far focus, comparing the data of focusing window areas between different frames, finding the clearest point, and then completing the focusing process. However, under the condition of continuous improvement of the resolution of a terminal camera such as a mobile phone camera, the terminal camera generally outputs an image in the form of full resolution while previewing the image, realizing zero delay function, therefore, the frame rate is lower. For example, the resolution of an existing camera is about 20 MP, but the frame rate of the camera is only 12 fps when the camera outputs an image in form of full resolution. Moreover, under low illumination environment, general processing on the image is to reduce the frame rate to obtain higher image brightness and lower noise. Therefore, the existing frame detection contrast ratio method has the disadvantage of poor performance, longer focus time and slower focus speed under the condition of previewing images in the form of full resolution by the single-camera and lower light environment, thus resulting in affecting the user experience.
Therefore, how to provide an image capturing terminal and an image capturing method for achieving quick autofocus, is a technical problem to be solved for the persons having ordinary skill in the art.